The M4 region of the Dictyosteluim genome contains two divergent transcription units; the 5'-ends of these genes are separated by 1.5 kb. These genes exhibit completely opposite modes of developmental regulation. M4-1 is expressed in vegetative cells but not in cells late in development. Its expression is repressed by developmental pulses of cAMP but is rapidly induced during dedifferentiation. Comparisons of nuclear and cytoplasmic RNA levels during development and dedifferentiation suggest that changes in expression of M4-1 are mediated at the level of transcription. In contrast, the expression of M4-4 increases during development. M4-4 contains a short, developmentally regulated repeat. The increase in expression of the repeated sequence during development parallels that of the unique portion of the M4-4 gene, suggesting coordinate regulation of the M4-4 repeat gene set.